This invention relates to a process for producing stable, sludge-free ammonium polyphosphate fertilizer solutions.
It is well known that ammonium phosphate fertilizers may be produced by the reaction of ammonia and phosphoric acid to produce ammonium phosphate. Ammonia for use in such reactions is readily available in comparatively pure form. However, phosphoric acid is available in several grades with varying degrees of purity. Because of the economics involved, wet-process grade phosphoric acid is ordinarily used in the production of fertilizers since wet-process acid is much less expensive than the other grades.
Characteristically, wet-process orthophosphoric acid contains relatively large amounts of dissolved impurities such as calcium sulfates, fluorides and fluorosilicates, aluminum, magnesium, iron and other metals. In commercial 54% P.sub.2 O.sub.5 acid, these impurities may comprise between about one and ten weight percent. When such orthophosphoric acid is treated with ammonia to form either mono- or di-ammonium phosphate, or a mixture of both, the metallic impurities are precipitated. The iron and aluminum impurities are particularly troublesome as they form gelatinous precipitates which are substantially impossible to separate from the aqueous phase by ordinary methods. When iron and aluminum are present in amounts greater than about three percent, the entire aqueous phase often gells into a solid mass.
The precipitates ordinarily do not interfere with the phytonutritive value of ammonium phosphate. Conversely, they are considered to have fertilizing value of their own. However, they tend to settle to the bottom of the storage vessels and clog pipe lines and other equipment used for applying the liquid product to the soil. Because of the precipitates formed by the impurities, ammonium phosphate made from wet-process acid for fertilizer purposes is often manufactured, marketed and applied in the solid form. However, evaporating the aqueous material to produce a dry product together with packaging and otherwise handling the product may substantially increase the cost of ammonium phosphate fertilizers manufactured from wet-process acid.
It has been discovered that some of the impurities which are found in wet-process orthophosphoric can be maintained in fertilizer solutions by the introduction of colloidal clays or other materials which maintain the precipitate in solution or suspension. However, the use of colloidal clays and the like is an expensive and involved process. It has also been discovered that reaction of ammonia with superphosphoric acid (phosphoric acid containing slightly more than 50% non-ortho forms of phosphoric acid such as pyro, meta, tri and higher forms) yields a high percentage of polyphosphates. High polysuperphosphoric acid, like the use of colloidal clays, however, is expensive.
Attempts have been made to reduce the concentration of precipitates in ammonium phosphate solutions produced by the ammoniation of wet-phosphoric acid or regular merchant grade phosphoric acid by increasing the polyphosphate content of the solutions. It has been found, however, that this technique has not been completely able to provide the sludge-free ammonium polyphosphate solutions even at polyphosphate conversion levels as high as 60%. Therefore, if the production of stable, sludge-free ammonium polyphosphate is desired, it has been necessary to utilize high polysuperphosphoric acid which as aforementioned is an expensive form of the acid. When the reaction with the anhydrous ammonia takes place in the ammonium phosphate production a considerable amount of heat is evolved. Consequently, the reaction system must be cooled down since the high temperatures involved will hydrolyze the desirable non-ortho phosphate back to orthophosphates according to the following reaction: EQU (NH.sub.4).sub.2 H.sub.2 P.sub.2 O.sub.2 + water .fwdarw. 2NH.sub.4 H.sub.2 PO.sub.4
it is an object of the present invention to produce clear, sludge-free ammonium polyphosphate solutions starting with wet-process phosphoric acid.
Another object of the invention is to produce ammonium polyphosphate solutions which do not require the heretofore high levels, e.g. 70% or more, of polyphosphate content to provide a stable, sludge-free solution.
Yet another object of the invention is to provide an ammonium polyphosphate solution of improved sequestration powers.